I'm not a expert, but this is what I've learned from years of church camps and Sunday services. After God created the Earth, Adam and Eve accepted the Snake of Evils temptations to eat fruit from the tree of knowledge. I think that was the first spark of sin, since God commanded them NOT to eat fruit from that tree. I believe that mans first sin was selfishness, because Adam and Eve literally had thousands of other trees to eat fruit from, but they still chose to eat from the one tree that was restricted. While Jesus was physically on Earth, he was never selfish. In fact, he was 100% the opposite. What I'm saying is that man "started" sin, and that Jesus has never sinned.